1. Field of the Invention
This invention relates to a process for producing polymeric material in which chemicals are mixed, heated and reacted to form a liquid stream of high viscosity. More particularly, the invention concerns an improvement in such a process wherein heat is supplied by microwave energy and the liquid stream of reactants is then passed through a vessel in plug flow such that substantially all volume elements of the stream are subjected to a uniform thermal history. The process is particularly suited for producing isocyanate-capped prepolymers that are used to make polyurethanes.
2. Description of the Prior Art
Chemical processes often require careful control of temperature and residence time of reactants to assure production of uniform products. It is particularly difficult to achieve uniform thermal histories when handling highly viscous liquid (e.g., liquid having a viscosity in a range of 1 to 10,000 poise). Typically, a highly viscous liquid passes through a pipe in laminar flow with a large velocity gradient between the wall and axis of the pipe. A volume element of stream near the wall requires several times as much time to pass through the pipe as does a volume element in the middle of the stream. Heating the stream from outside the pipe and through the pipe wall can add further thermal non-uniformities.
U.S. Pat. No. 3,461,261, Lewis et al, discloses resonant cavity microwave heating of materials such as threads, yarns and the like, axially disposed within an elongated cylindrical chamber. Resonant cavities with various transverse magnetic operating modes, including TM.sub.02n modes, are disclosed. However, the patent notes that transverse magnetic modes, such as TM.sub.02n, are inferior to other modes because of problems of mode separation. Risman et al., J. Microwave Power, 10 (3), 271-280, (1975), also discloses microwave heating of materials by use of a cylindrical wave guide applicator. Many transverse magnetic operating modes are disclosed. The final paragraph of the article states, with regard to a TM.sub.02n mode, "Heating of high-viscosity liquids is another application of great practical interest . . . it may be possible to achieve very even temperature profiles over the tube cross-section and thus very mild and rapid heat treatment. Further analysis is, however, required to verify this." No data are given to show that the possibility is actually achievable or how to do so.
Plug-flow reactors have been suggested for decreasing thermal history non-uniformities in liquid streams of chemical processes. For example, U.S. Pat. No. 3,486,865, Furusawa et al, discloses a plug-flow reactor having longitudinal and transverse stirring vanes and several compartments separated by transverse, preferably foraminous, rotating partitions. U.S. Pat. No. 4,424,301, Klippert et al, discloses a series of plug-flow reactors, one vertical and two inclined about 20 degrees to the horizontal. Reactors having multiple complicated horizontal members are difficult to clean especially if the reactors have been used for processing polymeric materials. Further, neither Furasawa et al nor Klippert et al discloses plug-flow reactors that can have a constant residence time with different throughputs.
The use of solutions of monofunctional amines in methylene chloride to clean apparatus used for the preparation of polyurethane sealants is disclosed in U.S. Pat. No. 4,675,126, Unger et al. However, the present inventors found that such solutions do not adequately clean equipment used for preparing spandex prepolymer.
A purpose of this invention is to provide a continuous process for making polymeric materials which subjects all volume elements in a viscous stream to substantially the same uniform history of temperature and residence time.